From 965da34de94cdb845b64bd676e45b260000cd09d Mon Sep 17 00:00:00 2001 From: Elliott Hughes Date: Tue, 18 Dec 2012 17:24:36 -0800 Subject: [PATCH] Update the JNI spec link, and remove a dead link. Also remove the duplicate copy of this file. There's a redirect in place from the old location. Bug: http://code.google.com/p/android/issues/detail?id=41623 Change-Id: I521b640fdef17c98582171d97214486d16e774e4 --- docs/html/guide/practices/app-design/jni.jd | 719 -------------------- docs/html/training/articles/perf-jni.jd | 7 +- 2 files changed, 3 insertions(+), 723 deletions(-) delete mode 100644 docs/html/guide/practices/app-design/jni.jd diff --git a/docs/html/guide/practices/app-design/jni.jd b/docs/html/guide/practices/app-design/jni.jd deleted file mode 100644 index ddfa0e3991f52..0000000000000 --- a/docs/html/guide/practices/app-design/jni.jd +++ /dev/null @@ -1,719 +0,0 @@ -page.title=JNI Tips -@jd:body - -
- -
- -

JNI is the Java Native Interface. It defines a way for managed code -(written in the Java programming language) to interact with native -code (written in C/C++). It's vendor-neutral, has support for loading code from -dynamic shared libraries, and while cumbersome at times is reasonably efficient.

- -

You really should read through the -JNI spec for J2SE 6 -to get a sense for how JNI works and what features are available. Some -aspects of the interface aren't immediately obvious on -first reading, so you may find the next few sections handy. -There's a more detailed JNI Programmer's Guide and Specification.

- - - -

JavaVM and JNIEnv

- -

JNI defines two key data structures, "JavaVM" and "JNIEnv". Both of these are essentially -pointers to pointers to function tables. (In the C++ version, they're classes with a -pointer to a function table and a member function for each JNI function that indirects through -the table.) The JavaVM provides the "invocation interface" functions, -which allow you to create and destroy a JavaVM. In theory you can have multiple JavaVMs per process, -but Android only allows one.

- -

The JNIEnv provides most of the JNI functions. Your native functions all receive a JNIEnv as -the first argument.

- -

The JNIEnv is used for thread-local storage. For this reason, you cannot share a JNIEnv between threads. -If a piece of code has no other way to get its JNIEnv, you should share -the JavaVM, and use GetEnv to discover the thread's JNIEnv. (Assuming it has one; see AttachCurrentThread below.)

- -

The C declarations of JNIEnv and JavaVM are different from the C++ -declarations. The "jni.h" include file provides different typedefs -depending on whether it's included into C or C++. For this reason it's a bad idea to -include JNIEnv arguments in header files included by both languages. (Put another way: if your -header file requires #ifdef __cplusplus, you may have to do some extra work if anything in -that header refers to JNIEnv.)

- - -

Threads

- -

All threads are Linux threads, scheduled by the kernel. They're usually -started from managed code (using Thread.start), -but they can also be created elsewhere and then attached to the JavaVM. For -example, a thread started with pthread_create can be attached -with the JNI AttachCurrentThread or -AttachCurrentThreadAsDaemon functions. Until a thread is -attached, it has no JNIEnv, and cannot make JNI calls.

- -

Attaching a natively-created thread causes a java.lang.Thread -object to be constructed and added to the "main" ThreadGroup, -making it visible to the debugger. Calling AttachCurrentThread -on an already-attached thread is a no-op.

- -

Android does not suspend threads executing native code. If -garbage collection is in progress, or the debugger has issued a suspend -request, Android will pause the thread the next time it makes a JNI call.

- -

Threads attached through JNI must call -DetachCurrentThread before they exit. -If coding this directly is awkward, in Android 2.0 (Eclair) and higher you -can use pthread_key_create to define a destructor -function that will be called before the thread exits, and -call DetachCurrentThread from there. (Use that -key with pthread_setspecific to store the JNIEnv in -thread-local-storage; that way it'll be passed into your destructor as -the argument.)

- - - -

jclass, jmethodID, and jfieldID

- -

If you want to access an object's field from native code, you would do the following:

- - - -

Similarly, to call a method, you'd first get a class object reference and then a method ID. The IDs are often just -pointers to internal runtime data structures. Looking them up may require several string -comparisons, but once you have them the actual call to get the field or invoke the method -is very quick.

- -

If performance is important, it's useful to look the values up once and cache the results -in your native code. Because there is a limit of one JavaVM per process, it's reasonable -to store this data in a static local structure.

- -

The class references, field IDs, and method IDs are guaranteed valid until the class is unloaded. Classes -are only unloaded if all classes associated with a ClassLoader can be garbage collected, -which is rare but will not be impossible in Android. Note however that -the jclass -is a class reference and must be protected with a call -to NewGlobalRef (see the next section).

- -

If you would like to cache the IDs when a class is loaded, and automatically re-cache them -if the class is ever unloaded and reloaded, the correct way to initialize -the IDs is to add a piece of code that looks like this to the appropriate class:

- -
    /*
-     * We use a class initializer to allow the native code to cache some
-     * field offsets. This native function looks up and caches interesting
-     * class/field/method IDs. Throws on failure.
-     */
-    private static native void nativeInit();
-
-    static {
-        nativeInit();
-    }
- -

Create a nativeClassInit method in your C/C++ code that performs the ID lookups. The code -will be executed once, when the class is initialized. If the class is ever unloaded and -then reloaded, it will be executed again.

- - -

Local and Global References

- -

Every argument passed to a native method, and almost every object returned -by a JNI function is a "local reference". This means that it's valid for the -duration of the current native method in the current thread. -Even if the object itself continues to live on after the native method -returns, the reference is not valid. -

This applies to all sub-classes of jobject, including -jclass, jstring, and jarray. -(The runtime will warn you about most reference mis-uses when extended JNI -checks are enabled.)

-

The only way to get non-local references is via the functions -NewGlobalRef and NewWeakGlobalRef. - -

If you want to hold on to a reference for a longer period, you must use -a "global" reference. The NewGlobalRef function takes the -local reference as an argument and returns a global one. -The global reference is guaranteed to be valid until you call -DeleteGlobalRef.

- -

This pattern is commonly used when caching a jclass returned -from FindClass, e.g.:

-
jclass localClass = env->FindClass("MyClass");
-jclass globalClass = reinterpret_cast<jclass>(env->NewGlobalRef(localClass));
- -

All JNI methods accept both local and global references as arguments. -It's possible for references to the same object to have different values. -For example, the return values from consecutive calls to -NewGlobalRef on the same object may be different. -To see if two references refer to the same object, -you must use the IsSameObject function. Never compare -references with == in native code.

- -

One consequence of this is that you -must not assume object references are constant or unique -in native code. The 32-bit value representing an object may be different -from one invocation of a method to the next, and it's possible that two -different objects could have the same 32-bit value on consecutive calls. Do -not use jobject values as keys.

- -

Programmers are required to "not excessively allocate" local references. In practical terms this means -that if you're creating large numbers of local references, perhaps while running through an array of -objects, you should free them manually with -DeleteLocalRef instead of letting JNI do it for you. The -implementation is only required to reserve slots for -16 local references, so if you need more than that you should either delete as you go or use -EnsureLocalCapacity/PushLocalFrame to reserve more.

- -

Note that jfieldIDs and jmethodIDs are opaque -types, not object references, and should not be passed to -NewGlobalRef. The raw data -pointers returned by functions like GetStringUTFChars -and GetByteArrayElements are also not objects. (They may be passed -between threads, and are valid until the matching Release call.)

- -

One unusual case deserves separate mention. If you attach a native -thread with AttachCurrentThread, the code you are running will -never automatically free local references until the thread detaches. Any local -references you create will have to be deleted manually. In general, any native -code that creates local references in a loop probably needs to do some manual -deletion.

- - -

UTF-8 and UTF-16 Strings

- -

The Java programming language uses UTF-16. For convenience, JNI provides methods that work with Modified UTF-8 as well. The -modified encoding is useful for C code because it encodes \u0000 as 0xc0 0x80 instead of 0x00. -The nice thing about this is that you can count on having C-style zero-terminated strings, -suitable for use with standard libc string functions. The down side is that you cannot pass -arbitrary UTF-8 data to JNI and expect it to work correctly.

- -

If possible, it's usually faster to operate with UTF-16 strings. Android -currently does not require a copy in GetStringChars, whereas -GetStringUTFChars requires an allocation and a conversion to -UTF-8. Note that -UTF-16 strings are not zero-terminated, and \u0000 is allowed, -so you need to hang on to the string length as well as -the jchar pointer.

- -

Don't forget to Release the strings you Get. The -string functions return jchar* or jbyte*, which -are C-style pointers to primitive data rather than local references. They -are guaranteed valid until Release is called, which means they are not -released when the native method returns.

- -

Data passed to NewStringUTF must be in Modified UTF-8 format. A -common mistake is reading character data from a file or network stream -and handing it to NewStringUTF without filtering it. -Unless you know the data is 7-bit ASCII, you need to strip out high-ASCII -characters or convert them to proper Modified UTF-8 form. If you don't, -the UTF-16 conversion will likely not be what you expect. The extended -JNI checks will scan strings and warn you about invalid data, but they -won't catch everything.

- - -

Primitive Arrays

- -

JNI provides functions for accessing the contents of array objects. -While arrays of objects must be accessed one entry at a time, arrays of -primitives can be read and written directly as if they were declared in C.

- -

To make the interface as efficient as possible without constraining -the VM implementation, the Get<PrimitiveType>ArrayElements -family of calls allows the runtime to either return a pointer to the actual elements, or -allocate some memory and make a copy. Either way, the raw pointer returned -is guaranteed to be valid until the corresponding Release call -is issued (which implies that, if the data wasn't copied, the array object -will be pinned down and can't be relocated as part of compacting the heap). -You must Release every array you Get. Also, if the Get -call fails, you must ensure that your code doesn't try to Release a NULL -pointer later.

- -

You can determine whether or not the data was copied by passing in a -non-NULL pointer for the isCopy argument. This is rarely -useful.

- -

The Release call takes a mode argument that can -have one of three values. The actions performed by the runtime depend upon -whether it returned a pointer to the actual data or a copy of it:

- - - -

One reason for checking the isCopy flag is to know if -you need to call Release with JNI_COMMIT -after making changes to an array — if you're alternating between making -changes and executing code that uses the contents of the array, you may be -able to -skip the no-op commit. Another possible reason for checking the flag is for -efficient handling of JNI_ABORT. For example, you might want -to get an array, modify it in place, pass pieces to other functions, and -then discard the changes. If you know that JNI is making a new copy for -you, there's no need to create another "editable" copy. If JNI is passing -you the original, then you do need to make your own copy.

- -

It is a common mistake (repeated in example code) to assume that you can skip the Release call if -*isCopy is false. This is not the case. If no copy buffer was -allocated, then the original memory must be pinned down and can't be moved by -the garbage collector.

- -

Also note that the JNI_COMMIT flag does not release the array, -and you will need to call Release again with a different flag -eventually.

- - - -

Region Calls

- -

There is an alternative to calls like Get<Type>ArrayElements -and GetStringChars that may be very helpful when all you want -to do is copy data in or out. Consider the following:

- -
    jbyte* data = env->GetByteArrayElements(array, NULL);
-    if (data != NULL) {
-        memcpy(buffer, data, len);
-        env->ReleaseByteArrayElements(array, data, JNI_ABORT);
-    }
- -

This grabs the array, copies the first len byte -elements out of it, and then releases the array. Depending upon the -implementation, the Get call will either pin or copy the array -contents. -The code copies the data (for perhaps a second time), then calls Release; in this case -JNI_ABORT ensures there's no chance of a third copy.

- -

One can accomplish the same thing more simply:

-
    env->GetByteArrayRegion(array, 0, len, buffer);
- -

This has several advantages:

- - -

Similarly, you can use the Set<Type>ArrayRegion call -to copy data into an array, and GetStringRegion or -GetStringUTFRegion to copy characters out of a -String. - - - -

Exceptions

- -

You must not call most JNI functions while an exception is pending. -Your code is expected to notice the exception (via the function's return value, -ExceptionCheck, or ExceptionOccurred) and return, -or clear the exception and handle it.

- -

The only JNI functions that you are allowed to call while an exception is -pending are:

- - -

Many JNI calls can throw an exception, but often provide a simpler way -of checking for failure. For example, if NewString returns -a non-NULL value, you don't need to check for an exception. However, if -you call a method (using a function like CallObjectMethod), -you must always check for an exception, because the return value is not -going to be valid if an exception was thrown.

- -

Note that exceptions thrown by interpreted code do not unwind native stack -frames, and Android does not yet support C++ exceptions. -The JNI Throw and ThrowNew instructions just -set an exception pointer in the current thread. Upon returning to managed -from native code, the exception will be noted and handled appropriately.

- -

Native code can "catch" an exception by calling ExceptionCheck or -ExceptionOccurred, and clear it with -ExceptionClear. As usual, -discarding exceptions without handling them can lead to problems.

- -

There are no built-in functions for manipulating the Throwable object -itself, so if you want to (say) get the exception string you will need to -find the Throwable class, look up the method ID for -getMessage "()Ljava/lang/String;", invoke it, and if the result -is non-NULL use GetStringUTFChars to get something you can -hand to printf(3) or equivalent.

- - - -

Extended Checking

- -

JNI does very little error checking. Errors usually result in a crash. Android also offers a mode called CheckJNI, where the JavaVM and JNIEnv function table pointers are switched to tables of functions that perform an extended series of checks before calling the standard implementation.

- -

The additional checks include:

- - - -

(Accessibility of methods and fields is still not checked: access restrictions don't apply to native code.)

- -

There are several ways to enable CheckJNI.

- -

If you’re using the emulator, CheckJNI is on by default.

- -

If you have a rooted device, you can use the following sequence of commands to restart the runtime with CheckJNI enabled:

- -
adb shell stop
-adb shell setprop dalvik.vm.checkjni true
-adb shell start
- -

In either of these cases, you’ll see something like this in your logcat output when the runtime starts:

- -
D AndroidRuntime: CheckJNI is ON
- -

If you have a regular device, you can use the following command:

- -
adb shell setprop debug.checkjni 1
- -

This won’t affect already-running apps, but any app launched from that point on will have CheckJNI enabled. (Change the property to any other value or simply rebooting will disable CheckJNI again.) In this case, you’ll see something like this in your logcat output the next time an app starts:

- -
D Late-enabling CheckJNI
- - - - - -

Native Libraries

- -

You can load native code from shared libraries with the standard -System.loadLibrary call. The -preferred way to get at your native code is:

- - - -

The JNI_OnLoad function should look something like this if -written in C++:

-
jint JNI_OnLoad(JavaVM* vm, void* reserved)
-{
-    JNIEnv* env;
-    if (vm->GetEnv(reinterpret_cast<void**>(&env), JNI_VERSION_1_6) != JNI_OK) {
-        return -1;
-    }
-
-    // Get jclass with env->FindClass.
-    // Register methods with env->RegisterNatives.
-
-    return JNI_VERSION_1_6;
-}
- -

You can also call System.load with the full path name of the -shared library. For Android apps, you may find it useful to get the full -path to the application's private data storage area from the context object.

- -

This is the recommended approach, but not the only approach. Explicit -registration is not required, nor is it necessary that you provide a -JNI_OnLoad function. -You can instead use "discovery" of native methods that are named in a -specific way (see the JNI spec for details), though this is less desirable because if a method signature is wrong you won't know -about it until the first time the method is actually used.

- -

One other note about JNI_OnLoad: any FindClass -calls you make from there will happen in the context of the class loader -that was used to load the shared library. Normally FindClass -uses the loader associated with the method at the top of the interpreted -stack, or if there isn't one (because the thread was just attached) it uses -the "system" class loader. This makes -JNI_OnLoad a convenient place to look up and cache class -object references.

- - - -

64-bit Considerations

- -

Android is currently expected to run on 32-bit platforms. In theory it -could be built for a 64-bit system, but that is not a goal at this time. -For the most part this isn't something that you will need to worry about -when interacting with native code, -but it becomes significant if you plan to store pointers to native -structures in integer fields in an object. To support architectures -that use 64-bit pointers, you need to stash your native pointers in a -long field rather than an int. - - - -

Unsupported Features/Backwards Compatibility

- -

All JNI 1.6 features are supported, with the following exception:

- - -

For backward compatibility with older Android releases, you may need to -be aware of:

- - - - -

FAQ: Why do I get UnsatisfiedLinkError?

- -

When working on native code it's not uncommon to see a failure like this:

-
java.lang.UnsatisfiedLinkError: Library foo not found
- -

In some cases it means what it says — the library wasn't found. In -other cases the library exists but couldn't be opened by dlopen(3), and -the details of the failure can be found in the exception's detail message.

- -

Common reasons why you might encounter "library not found" exceptions:

- - -

Another class of UnsatisfiedLinkError failures looks like:

-
java.lang.UnsatisfiedLinkError: myfunc
-        at Foo.myfunc(Native Method)
-        at Foo.main(Foo.java:10)
- -

In logcat, you'll see:

-
W/dalvikvm(  880): No implementation found for native LFoo;.myfunc ()V
- -

This means that the runtime tried to find a matching method but was -unsuccessful. Some common reasons for this are:

- - -

Using javah to automatically generate JNI headers may help -avoid some problems. - - - -

FAQ: Why didn't FindClass find my class?

- -

Make sure that the class name string has the correct format. JNI class -names start with the package name and are separated with slashes, -such as java/lang/String. If you're looking up an array class, -you need to start with the appropriate number of square brackets and -must also wrap the class with 'L' and ';', so a one-dimensional array of -String would be [Ljava/lang/String;.

- -

If the class name looks right, you could be running into a class loader -issue. FindClass wants to start the class search in the -class loader associated with your code. It examines the call stack, -which will look something like: -

    Foo.myfunc(Native Method)
-    Foo.main(Foo.java:10)
-    dalvik.system.NativeStart.main(Native Method)
- -

The topmost method is Foo.myfunc. FindClass -finds the ClassLoader object associated with the Foo -class and uses that.

- -

This usually does what you want. You can get into trouble if you -create a thread yourself (perhaps by calling pthread_create -and then attaching it with AttachCurrentThread). -Now the stack trace looks like this:

-
    dalvik.system.NativeStart.run(Native Method)
- -

The topmost method is NativeStart.run, which isn't part of -your application. If you call FindClass from this thread, the -JavaVM will start in the "system" class loader instead of the one associated -with your application, so attempts to find app-specific classes will fail.

- -

There are a few ways to work around this:

- - - - -

FAQ: How do I share raw data with native code?

- -

You may find yourself in a situation where you need to access a large -buffer of raw data from both managed and native code. Common examples -include manipulation of bitmaps or sound samples. There are two -basic approaches.

- -

You can store the data in a byte[]. This allows very fast -access from managed code. On the native side, however, you're -not guaranteed to be able to access the data without having to copy it. In -some implementations, GetByteArrayElements and -GetPrimitiveArrayCritical will return actual pointers to the -raw data in the managed heap, but in others it will allocate a buffer -on the native heap and copy the data over.

- -

The alternative is to store the data in a direct byte buffer. These -can be created with java.nio.ByteBuffer.allocateDirect, or -the JNI NewDirectByteBuffer function. Unlike regular -byte buffers, the storage is not allocated on the managed heap, and can -always be accessed directly from native code (get the address -with GetDirectBufferAddress). Depending on how direct -byte buffer access is implemented, accessing the data from managed code -can be very slow.

- -

The choice of which to use depends on two factors:

-
    -
  1. Will most of the data accesses happen from code written in Java - or in C/C++? -
  2. If the data is eventually being passed to a system API, what form - must it be in? (For example, if the data is eventually passed to a - function that takes a byte[], doing processing in a direct - ByteBuffer might be unwise.) -
- -

If there's no clear winner, use a direct byte buffer. Support for them -is built directly into JNI, and performance should improve in future releases.

diff --git a/docs/html/training/articles/perf-jni.jd b/docs/html/training/articles/perf-jni.jd index 26b06b4f1a645..cee93c54dc586 100644 --- a/docs/html/training/articles/perf-jni.jd +++ b/docs/html/training/articles/perf-jni.jd @@ -31,12 +31,11 @@ page.title=JNI Tips code (written in C/C++). It's vendor-neutral, has support for loading code from dynamic shared libraries, and while cumbersome at times is reasonably efficient.

-

You really should read through the -JNI spec for J2SE 6 +

If you're not already familiar with it, read through the +Java Native Interface Specification to get a sense for how JNI works and what features are available. Some aspects of the interface aren't immediately obvious on -first reading, so you may find the next few sections handy. -There's a more detailed JNI Programmer's Guide and Specification.

+first reading, so you may find the next few sections handy.